In anode bodies of commercially available electrolytic capacitors, tantalum or aluminum is widely used. In the surface of such an anode body, a coating film (chemical conversion coating film) composed of an oxide of the anode material is formed by anodization (chemical conversion treatment). The chemical conversion coating film serves as the dielectric layer of the electrolytic capacitor. Since the performance characteristics of aluminum electrolytic capacitors highly differ from those of tantalum electrolytic capacitors, they are applied to different respective uses.
In the meantime, niobium metals are known to have physical and chemical properties similar to those of tantalum metals. Niobium is abundant as a mineral resource compared to tantalum and is inexpensive. In addition, niobium pentoxide has a high dielectric constant compared to other metal oxides. Accordingly, niobium has been studied to replace tantalum used in tantalum electrolytic capacitors.
However, a niobium oxide coating film obtained by chemically converting a niobium anode body is unstable compared to a tantalum oxide coating film. In particular, the thickness of a niobium oxide coating film per formation voltage is twice that of a tantalum oxide coating film, and the strain occurring with the growth of a niobium oxide coating film is also twice that of a tantalum oxide coating film. Therefore, the breakdown voltage per unit thickness of the niobium oxide coating film is half that of the tantalum oxide coating film.
Furthermore, niobium oxide contains a nonstoichiometric lower oxide, which is not present in tantalum oxide. It is thought that this encourage diffusion of oxygen in a dielectric layer, imparts semiconducting properties to the dielectric layer, and increases leakage current.
Thus, the niobium oxide coating film has unstable characteristics. However, the niobium electrolytic capacitors have a possibility of showing characteristics superior to those of the tantalum electrolytic capacitors. Accordingly, many studies have been further conducted.
For example, Patent Document 1 describes a method for manufacturing a niobium electrolytic capacitor by chemically converting a niobium sintered body or niobium foil in an electrolyte aqueous solution containing chlorine ions at a solution temperature of −15° C. to 100° C. and subsequently performing aging in an electrolytic solution substantially not containing halogen ions. This electrolyte aqueous solution is prepared by dissolving a chloride based electrolyte, such as hydrogen chloride, a metal chloride, or a chloride of ammonium or amine, in water.
Patent Document 2 describes manufacturing an electrolytic capacitor anode by sintering flaked niobium powder in vacuum and anodizing the sintered body in an 0.1 wt % aqueous solution of phosphoric acid. The temperature of the phosphoric acid aqueous solution during the anodization is not specifically disclosed in Patent Document 2, but it is supposed that the temperature is a level at which conventional chemical conversion is performed by a person skilled in the art, that is, about 60° C. to 90° C.
Patent Document 3 proposes a process for chemical conversion by immersing a niobium anode body in an aqueous solution (chemical conversion solution) containing at least one acid selected from phosphoric acid, nitric acid, and sulfuric acid and performing chemical conversion at a solution temperature of not lower than the freezing point and not higher than about 40° C. It is disclosed that the freezing point of the chemical conversion solution slightly varies depending on the kind and the concentration of the solute, but is about 0° C. (or a temperature slightly lower than about 0° C.). In the examples of Patent Document 3, the solution temperature during chemical conversion is set at 5° C. to 40° C.
Patent Document 4 describes a method for manufacturing a solid electrolytic capacitor in which an oxide coating film is formed in a surface of a porous sintered body of tantalum, which is a metal exhibiting valve action, by immersing the sintered body in an aqueous solution containing hydrogen peroxide and phosphoric acid to anodize the sintered body. The chemical conversion solution temperature during the anodization is not specifically disclosed in Patent Document 4, but it is supposed that the temperature is a level at which conventional chemical conversion is performed by a person skilled in the art, that is, about 60° C. to 90° C.
Patent Document 5 discloses a method for manufacturing an electrolytic capacitor in which a dielectric coating film layer is formed in the surface of a niobium or niobium-based alloy anode body, and then a first conductive polymer layer of polypyrrole or a polypyrrole derivative is formed on the dielectric coating film layer by immersing the anode body provided with the dielectric coating film layer in a solution comprising 0.7 to 10 wt % of hydrogen peroxide and 0.3 to 3 wt % of sulfuric acid and water as a main solvent, pulling up the anode body and exposing it to vapor of pyrrole or a pyrrole derivative. The dielectric coating film layer is formed by sintering niobium powder to a porous anode element and immersing the porous anode element in an aqueous solution of phosphoric acid at 5° C. for chemical conversion treatment at a voltage of 38 V.
Patent Document 6 describes a method for manufacturing an anode for an electrolytic capacitor by immersing a metal exhibiting valve action in an electrolytic solution to anodize the metal at 40° C. It is shown that when tantalum powder is used as the metal exhibiting valve action, the electrolytic solution composed of ethylene glycol or polyethylene glycol, deionized water, and phosphoric acid is used.
Patent Document 7 discloses an anodization electrolytic solution for forming a dielectric oxide on a metal exhibiting valve action. The anodization electrolytic solution contains water; oxo acid of phosphorus or its salt; at least one selected from the group consisting of inorganic acids, salts of inorganic acids, carboxylic acid, carboxylates and mixtures thereof; and a protic solvent. As examples of the protic solvent when the metal exhibiting valve action is tantalum, alkylene glycol and polyalkylene glycol are disclosed. The chemical conversion solution temperature during the anodization is not specifically disclosed, but it is supposed that the temperature is a level at which conventional chemical conversion is performed by a person skilled in the art, that is, about 60° C. to 90° C.